Polyoxymethylene (POM, also known as polyacetal) has excellent tribological characteristics, hardness, stiffness, moderate toughness, low coefficient of friction, good solvent resistance, and the ability to crystallize rapidly, making polyoxymethylene resin compositions useful for preparing articles for use in many demanding applications. However, during melt-processing, polyoxymethylenes can degrade and release formaldehyde. For this reason, formaldehyde evolution, measured as thermally evolved formaldehyde (TEF) is often used to determine the heat stability of polyoxymethylene compositions. Other methods for assessing the heat stability of polyoxymethylene include the measurement of weight loss in air over aging at elevated temperatures for prolonged periods, such as 500 to 1000 hrs and the observation of degradative coloration of the resin compositions using the Yellowness index, as defined in ASTM E31 3-73 (D 1925).
It would be desirable to have polyoxymethylene compositions that exhibited good or excellent thermal stability during melt-processing and aging, as assessed by one or more of the above-mentioned methods.
Int'l App. Pub. No. WO 941488 discloses polyacetal compositions including cyclodextrin as a thermal stabilizer. U.S. Pat. No. 5,011,890 discloses polyacetal compositions including polymers having formaldehyde reactive nitrogen groups such as polyacrylamide. U.S. Pat. No. 5,106,888 discloses polyacetal compositions including microcrystalline cellulose (MCC) as a thermal stabilizer with polyamide. The use of epoxidized fatty acid stabilizer as polyoxymethylene stabilizers has been disclosed in U.S. Pat. App. Pub. No. 2005/0288438.